Sealed wave guide window



Nov 1, 1.960 R. s. SYMONS ETAL 2,953,834

SEALED WAVE GUIDE WINDOW Filed June 13. 1956 Attorney SEALED WAVE GUIDE WINDQW Robert S. Symons, Menlo Park, and Arthur E. Schoennauer, Palo Alto, Calif, assignors to Varian Associates, San Carlos, Calif a corporation of California Filed June 13, 1956, Ser. No. 591,086

18 Claims. (Cl. 333-98) This invention relates in general to waveguide transmission lines and more specifically to wave permeable gas-tight partitions therein, called windows. In many high frequency systems it is often found that it is necessary to pass wave energy through a waveguide having portions of its length operating at a reduced pressure or wherein one or more portions of the guide contain different gaseous materials at different pressures. The present invention is useful for providing a relatively broad band, high power gas-tight wave permeable window assembly useful in such high frequency systems.

In thev hollow waveguide transmission art it has been common practice to insert within the hollow waveguides various types of wave permeable gas-tight windows having a multitude of physical configurations and chemical compositions. For example, such gas-tight windows have taken the form of perpendicular transverse mica walls, ceramic cones, slanted circular ceramic. discs, slanted transverse mica walls, and walls having stairstep configurations, to name a few. Universally, these prior art Window designs have been plagued with a common problem, namely, discontinuities and irregularities in the guide through which the electromagnetic energy must propagate introduce uncanceled reflections and perturbations in the, electromagnetic waves resulting in standing waves thereby decreasing the transmission efficiency and possibly producing exessively high electric fields in the waveguide or elsewhere in the system. The undesirable reflections can arise due to discontinuities in the transmission line or waveguide associated with the junction between window and waveguide as well as from the opposite faces of the window or wave permeable member itself.

A large variety of prior art window configurations exemplify the attempts by the industry to minimize the reflected energy. In generalthe approach has involved designing the window and associated junction such that reflecting irregularities or discontinuities are successively arranged along the length of the waveguide in sucha manner that a reflection from one irregularity is canceled by the reflection from another irregularity or plurality of irregularities. For example, in the case of thick windows, reflections have heretofore been suppressed by providing a critical thickness of window related to the wavelength, whereby reflections from opposite faces of the window would mutually cancel out. As might be expected, these prior art window designs have been relatively narrow band devices. This is because the irregularities are arranged within the guide to produce reflections that cancel each other out for a given wavelength or frequency. However, for energy of a frequency differing substantially from the central or reference wavelength, satisfactory canceling does not take place resulting in a loss of transmitted power manifested by voltage standing waves in the transmission line betweenthe source and window assembly.

The. present invention: provides a novel gas-tight window assembly whereby the reflections from various dis- States Patent O Patented Nov. 1, 1960 "ice continuities and irregularities associated with the novel window and associated waveguide junction cancel out over a broad band of frequencies. Moreover, the present window design will allow the transmission of relatively large amounts of power therethrough without arcing over or otherwise breaking down.

The principal object of the present invention is to provide a novel improved high power, broad band gastight window assembly for hollow waveguide transmission lines.

One feature of the present invention is the provision of a gas-tight wave permeable window secured in a first section of waveguide and having joined to the guide on opposite sides of the window, by abrupt transitions, sections of lower impedance guide.

Another feature of the present invention is the provision of a circular waveguide having secured therein a circular window.

Another feature of the present invention is the provision of a circular waveguide having secured therein a circular ceramic window and being coupled to lower impedance rectangular waveguides on opposite sides of said window.

Another feature of the present invention is the provision of a circular waveguide having secured therein substantially at its midpoint a circular ceramic window and said window being at the point of maximum electric field in said circular waveguide.

Another feature of the present invention is the provision of a relatively thin walled yieldable cup member serving to hold the wave permeable window,member therewithin and a relatively thick walled cup member enveloping said first thin walled member. and spaced apart therefrom to allow for the thermal expansion and contraction of said window member and'said thin Walled member in use. U

Another feature of the present invention is a half wave choke means communicating with the novel window assembly whereby the Window holding member may flex to allow for the thermal expansion and contraction of the structure in use.

These and other features and advantages of the present invention will be more apparent after a perusal of the following specification taken in connection with the accompanying drawings wherein,

Fig. l is a perspective View, partially cut away; of a window assembly showing atphy'sical embodiment of the present invention,

Fig. 2 is a reduced longitudinal cross section'of the structure of Fig. 1,

Fig. 3 is a longitudinal cross sectionof a novet window assembly forming another embodiment of the present invention, and- Fig. 4 is: a schematic electrical equivalent circuit of the structure of Figs. 1, 2 and 3-.

Referring now to Figs; 1 and 2 there is shownone embodiment of the present invention. A section of cir-' cular waveguide 1 carries: transversely therein a gas-tight wave permeablewindow E as of, for example, aluminum oxide (A1 0 or 'qu'artz or other dielectric, as desired; Closing off both ends of the circular waveguid'er L are two apertured transverse conductive metallic wall rnem bers 3.- Two sections of. rectangular waveguide 4 are coupled to the circular waveguide through the apertured end walls 3 and are securedito said end walls 3 as; for example, by brazing.

In operation energy enters at terminal a. and: propagates through the structure and outof terminal b. However, due to electrical symmetry ofthe'apparatusthe device is electrically reciprocal, thatis, the. electrical: prop.- erties of the device are substantially identicalno-matter whether energy enters terminal a or b.

The novel window 2 and associated wave guide junctions are designed such that abrupt transitions c and d between rectangular and circular waveguides are made electrically in the order of /2 of a wavelength apart at the center frequency of the pass band, when the window is in place, modified to compensate for the effects of the discontinuities. A structure which is electrically one wavelength long is defined as one which causes a phase shift of 21r radians in a wave propagating therethrough. Also, the window member itself is substantially equally spaced from both transitions and d and will be positioned within the circular waveguide 1 at a situs of maximum electric field strength.

Although the device will operate at electrical lengths, between the junctions, in the order of any odd number n of half-wavelengths, the higher values of n will make the device more narrow in bandwidth than for a value of n=1. It is believed that the wide bandwidth characteristics of the present device are attributable to the changes in the unmatched net discontinuity capacitance formed by the abrupt waveguide transitions 0 and d and impedance ratios of the junctions with frequency, whereby reflections are canceled out over a broad band of frequencies. It has been found that such a window and associated junctions, when properly designed, will provide a bandwidth of approximately 30%. Moreover, the present window design is capable of transmitting relatively large amounts of power without breaking down or arcing over.

Referring now to Fig. 3 there is shown another embodiment of the present invention. In this embodiment the electrical characteristics are the same as those of the structure of Fig. 2 but the apparatus has been altered slightly for ease of manufacture and to provide a more flexible holder for the wave permeable sealing window or member to allow for the thermal expansion and contraction of the wave permeable member and associated holder in use. Specifically, a first section of rectangular waveguide 5 is secured, as by brazing, to an apertured metallic conductive cup member 6. A flange-like lip portion 7 is provided on the enlarged open end of cup member 6. A second apertured cup member 8 as of, for example, copper is made of a thin wall construction. Cup member 8 is carried within outer cup member 6 by a flange-like lip portion 9. Lip portion 9 is secured to similar lip-like portion 7 of outer cup member 6 as, for example, by brazing thereby forming a gas-tight seal therebetween.

The outside diameter of cup member 8 is slightly less than the inside diameter of outer cup member 6 and the bottom portion of inside cup member 8 is slightly spaced from the bottom portion of outer cup member 6 whereby a cup-like space is defined by the boundaries of cup members 8 and 6. The extent of this space from the junction of members 5 and 6 is approximately a half wavelength thereby providing a half wave choke, which presents a corresponding series short circuit to the wave energy propagating down waveguide 5. The half wave choke consists of a MA low impedance radial section connected to a MA higher impedance circular section in order to obtain broad band performance. Although the window structure is electrically approximately a half wavelength long, it is short enough physically, due to the high dielectric constant of the window, to be surrounded with the X/ 4 circular choke section.

The half wave choke has been shown directly communicating with the rectangular guide 5 supbstantially at the junction of waveguides 5 and 8. However, the placement of this choke is a matter of convenience and might just as well have been positioned in other locations wherein it could serve to provide a free end portion for the window holding member 8. For example, it could have been placed such as to directly communicate only with window holding member 8.

A gas-tight wave permeable window member 11 is secured within cup member 8 transversely therein substantially at the midpoint thereof. The relatively thin wall construction of inner cup member 8 provides a flexible support for wave permeable window 11 to allow for the thermal expansion and contraction of window member 11 and inner cup member 8, in use, without breaking the seal between these members. The large open end portions of cup members 8 and 6 are closed off by a section of flanged rectangular waveguide 12.

The electrical operation of the embodiment shown in Fig. 3 is substantially the same as for the structure shown in Fig. 2. That is, the interior dimensions of the section of rectangular waveguide 5 and 12 and circular cup member 8 will be substantially the same as the interior dimensions of the structure of Fig. 2. However, the structural arrangement of the elements has been altered slightly to provide ease of manufacture and to allow for the thermal characteristics of the device.

In designing the novel window and associated waveide structure of the present invention, it has been found that the final dimensions may he arrived at through a process of cut and try. An approximate design method has been worked out which will allow the calculation of approximately the correct dimensions of the various parts. Then by cut and try methods, starting from the calculated values, one is able to arrive at a final design yielding the broad bandwidth characteristics set forth above.

The approximate design method can be derived from Fig. 4 wherein there is shown a schematic equivalent circuit diagram of the novel window and associated waveguide junctions. By reference to Fig. 4 it can be seen that the abrupt transitions between rectangular and circular waveguide can be represented by unmatched net shunting capacitors C and C shunting a two wire transmission line, of a characteristic impedance Z The portions of circular waveguide on either side of the circular window 2 can be represented by corresponding lengths of higher characteristic impedance Z That portion of the circular waveguide 1 which is filled with the wave permeable window member 2 may be considered as a short length of circular waveguide of characteristic impedance Z Z may be calculated knowing the dielectric constant of the wave permeable window member 2.

The characteristics of the capacitive discontinuities between the round and the rectangular waveguides may be measured and the characteristics of the junction between air filled and dielectric filled round waveguide may be calculated. By using this information approximate dimensions may be arrived at using a Smith chart and standard matching design procedures.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high frequency gas tight wave permeable window assembly dimensioned for passing wave energy therethrough over a certain passband of frequencies including; a first tubular waveguide member having a first characteristic guide impedance; second and third tubular waveguide members having characteristic guide impedances substantially less than said first characteristic guide impedance; means forming abrupt waveguide transitions connecting said second and third waveguide members to said first waveguide member substantially at the ends of said first waveguide member; said abrupt waveguide transition means forming spaced apart net capacitive discontinuities in a transmission line formed by said connected first, second and third waveguide members; a gas tight wave permeable window member disposed transversely of and Within said first waveguide member approximately midway the length thereof; the thickness of said wave permeable window member being substantially less than one-half an electrical wavelength at the center frequency of the passband of the wave permeable window assembly; and the length of said first waveguide member with said wave permeable window in place being approximately n/2 electrical wavelengths long at the center frequency of the certain passband of said window assembly, where n can be any odd integer value.

2. The apparatus according to claim 1 wherein said first tubular waveguide is a circular waveguide.

3. The apparatus according to claim 1 wherein said second and third tubular waveguides are rectangular gu d 4. The apparatus according to claim 1 wherein said wave permeable member includes a circular alumina ceramic disk.

5. The apparatus according to claim 4 where n is equal to one.

6. A high frequency gas tight wave permeable window assembly dimensioned for passing wave energy therethrough over a certain passband of frequencies including; a first length of substantially circular waveguide having a certain characteristic guide impedance; second and third lengths of substantially rectangular waveguide having substantially equal characteristic guide impedances which are substantially less than said first characteristic guide impedance; means forming abrupt waveguide transitions connecting said first length of circular waveguide to said second and third lengths of rectangular waveguide and disposed substantially at the ends of said first waveguide; said abrupt transition means forming net capacitive discontinuities in a transmission line formed by said connected first, second and third waveguide lengths; a substantially circular alumina ceramic member sealed transversely of and within said first length of waveguide and forming a gas tight wave permeable partition thereacross; said ceramic member being disposed substantially midway the length of said first length of waveguide and being substantially less than one-half an electrical wavelength thick at the center frequency of the passband of the window assembly; and said first length of circular waveguide, with said ceramic member in place and connected to said second and third lengths of waveguide being approximately n/2 wavelengths in electrical length at the center frequency of the certain passband of the window assembly, where n can have any odd integer value.

7. The apparatus according to claim 6 including; means forming a Waveguide choke connected into said transmission line substantially at one of said abrupt transitions in said transmission line for presenting to said transmission line a very low impedance to wave energy passing therethrough.

8. A gas tight wave permeable window assembly dimensioned for passing wave energy therethrough over a certain passband of frequencies including; a first length of tubular waveguide having a first characteristic guide impedance; a wave permeable window member sealed transversely of and within said first length of waveguide and forming a gas tight partition thereacross; a second length of tubular waveguide having a second characteristic guide impedance substantially less than said first characteristic guide impedance; a first abrupt waveguide transition connecting said first and second tubular waveguides; means forming a flange carried from said first waveguide for connecting said first waveguide to a third length of tubular waveguide via a second abrupt waveguide transition; said third waveguide having a third characteristic guide impedance substantially less than said first characteristic guide impedance; said first and second abrupt transitions forming net capacitive discontinuities in a transmission line formed when said first, second and third waveguides are connected; said wave permeable window member having a thickness substantially less than one half an electrical wavelength at the center frequency of the passband of the window assembly; and said first length of Waveguide, with said window member in place, having a length between the center of said window member and said first abrupt waveguide transition of approximately n/ 4 wavelengths in electrical length at the center frequency of the certain passband of the window assembly, and the center of said window member also being disposed approximately 11/ 4 electrical wavelengths from said second abrupt Waveguide transition in the transmission line formed when said first, second and third waveguides are connected, where n can have any odd integer value.

9. A high frequency gas-tight wave permeable window assembly comprising a first length of waveguide means having a certain guide impedance Z a gas-tight wave permeable window means carried transversely within said first length of waveguide substantially at the midpoint thereof, second waveguide means coupled to said first waveguide means substantially at the ends of said first waveguide means, abrupt transition means forming a transition between said first and said second waveguide means, said second waveguide means having lower rimpedance than said first waveguide means, and choke means communicating with one of said waveguide means and closely spaced to said abrupt transition means whereby a portion of said first waveguide means is provided with a free end portion and is made thin and flexible to allow for the thermal contraction and expansion of said window means in use.

10. In an apparatus as claimed in claim 9 wherein said first waveguide means comprises a length of circular waveguide of electrical length in the order of n/2 wavelengths long compensated for the effects of the discontinuities introduced by said abrupt transition means and said window means, where n can be any odd integer value.

11. In an apparatus as claimed in claim 10 wherein said second waveguide means comprise lengths of rectangular waveguide.

12. In an apparatus as claimed in claim 11 wherein said choke means is disposed substantially at a junction between said first and said second waveguide means, and wherein said waveguide choke means is electrically substantially n/2 wavelengths long whereby said choke means presents to the junction a very low impedance, where n can be any integer value.

13. In an apparatus as claimed in claim 12 wherein said waveguide choke means comprises a two section broad band radial choke means.

14. In an apparatus as claimed in claim 13 wherein said two section choke means comprises a first radial section substantially wavelength long electrically and a second circular section of substantially higher impedance than said first section and substantially wavelength in electrical length whereby a broad band choke is provided.

15. In an apparatus as claimed in claim 12 wherein said second waveguide means includes an enlarged portion which envelopes said first waveguide means and said window means and makes a gas-tight seal with said first waveguide means.

16. A high frequency gas-tight Wave permeable window assembly comprising, a first section of rectangular waveguide having an enlarged flanged hollow bulbous end thereto, a flanged cup member having an aperture in the bottom portion thereof secured to and carried within the bulbous portion of said first rectangular waveguide in spaced apart relation thereto, a gas-tight wave permeable window member secured to and carried transversely within said cup member, and a second flanged section of rectangular waveguide secured to and closing oif the hollow bulbous portion of said first section of waveguide.

17. In a gas-tight wave permeable window assembly as claimed in claim 16 wherein said flanged cup member is of thinner wall construction than the bulbous portion of said first rectangular waveguide whereby the thermal expansion and contraction of the window member may be allowed in use without placing undue stress on the window member or cup member.

18. A high frequency gas-tight permeable window assembly comprising a first length of waveguide means having a certain guide impedance Z a gas-tight wave permeable window means carried transversely within said first length of waveguide substantially at the midpoint thereof, said wave permeable window means vhaving a thickness small compared to a half wavelength, second waveguide means coupled to first waveguide means substantially at the ends of said first waveguide means, abrupt transition means forming a transition between said first and second waveguide means, said second waveguide means having lower impedance than said first waveguide means, a flexible member supporting said Window means within said first length of Waveguide, and choke means communicating with one of said waveguide means and being closely spaced to said abrupt transition means and presenting a series short circuit to R.F. energy between said flexible window support and one of said waveguides whereby said Window supporting member may be made thin and flexible to allow for the thermal contraction and expansion of said window means in use.

References Cited in the file of this patent UNITED STATES PATENTS Tonks Sept. 17, 1946 Ring Aug. 12, 1947 Johnson et al Nov. 28, 1950 Cohn Feb. 2, 1954 Kline et al. Dec. 28, 1954 Remond May 8, 1956 

